Range Management (RM) is a business process for managing daily work in a manufacturing line, driving reductions in cycle time, inventory, and cost, and driving gains in output and throughput. A standard RM system is focused on delivering work in progress (WIP) on a known route at a known speed to meet customer deliveries on schedule. In a typical RM system, each known route is assigned to a flow. There are usually several flows, each containing a group of routes, which will cover all routes in the manufacturing line. The flows are then broken into a number of ranges, with the goal of creating equal cycle time ranges. Each range contains a collection of processes with raw process times which will ideally sum to one day of cycle time. The end goal is for each product to experience “1 day of cycle time,” at the defined speed, in each day (24 hours).
One of the most important aspects of implementing an RM system is setting the range boundaries properly. The range boundaries are the foundation of the system—if they do not divide the flows into equitable portions, the WIP will not move smoothly, resulting in artificially faster or slower regions within each flow. Since each range will necessarily contain operations from several different areas of the manufacturing line, a methodology for determining the appropriate content of each range is often not obvious or straightforward.
One approach is to divide each flow into “best possible” equal raw process time (RPT) units without consideration of range content, possibly followed by slight manual adjustments of the range boundaries. One metric that can be used to judge how equitable the range divisions are is a histogram plot of the range RPT sums for each flow. A well divided set of ranges will have a peak in the distribution very close to the ideal target (RPTsum,target=24 hours/flow X factor), and a very narrow distribution around this peak.
A tight and well focused distribution of the range RPT sums, however, is a necessary but not sufficient condition of a well designed RM system. Dividing operations into ranges based on RPT-based considerations alone ignores the various content and behavior of the operations. Complexities in the flow of WIP must also be considered. Examples of complex WIP flow regions include: (1) rework-sensitive operations; (2) “groups” of operations treated as one logical unit of work by the manufacturing personnel; (3) operations that may be “sampled” at some frequency, such as short measurement operations following process operations; and (4) multi-operation time-sensitive regions.
In another typical RM system, the range boundaries are instead set manually by aligning common process regions that occur in somewhat different locations for each flow. This process will generate the most “human understandable” range boundaries, but it will necessarily produce gaps between the ranges within each flow, since the collection of manufacturing process routes executed in parallel are seldom identical. It is also very difficult to assure equitable distribution of the RPT sum for each range using this approach. Furthermore, this manual process is very time consuming to execute and requires a non-trivial investment in manpower. This presents a problem since the range break process must be executed at some frequency in order to account for changes that will almost always occur to product routings over time.
In another aspect of a standard RM scheme, each flow is broken into ranges using a single X factor (which will be understood by one of skill in the art as cycle-time multiplier, and which is equivalent to cycle-time performance divided by raw process time), reflecting the overall X factor at which the product is planned to run. In reality however, parts flow at different rates through different sections of the line depending on the tools used in that section of the line. Depending on various factors like costs, projected loading and availability, tools are planned to be utilized at different rates. Using a single X factor to break a flow into ranges results in the creation of some ranges in which targets are consistently easily met, and others that are nearly impossible to complete in a day, depending on the toolsets contained in that range. This is a significant problem because real problems hampering line performance are not identified; instead, effort is expended in analysis of areas that are not major concerns.